The present invention relates to a silicone composition for forming a release coating and in particular to a non-solvent type silicone composition for forming a cured release coating for adhesive substances on the surfaces of various sheet-like substrates such as paper, synthetic resin films, and metal foil.
Known in the art is a method of obtaining a material that imparts release properties for adhesive substances, the material being produced by applying a curable silicone composition on the surface of a sheet-like substrate such as paper, laminated paper, synthetic resin film, and metal foil and then forming a cured coating by heating the aforementioned composition. It is also known that the main components of the silicone composition used in the above method for forming a cured coating are an organohydrogenpolysiloxane and an organopolysiloxane with alkenyl groups. The composition is subjected to an addition reaction in the presence of a platinum-type, catalyst to cure. For example, Japanese Patent Application Laid-Open Number (Kokai) Hei 9-125,004 describes a composition which consists of an organopolysiloxane having a high degree of polymerization containing hexenyl groups, an organohydrogenpolysiloxane, a platinum catalyst, and an organic solvent. Since the diorganopolysiloxane with hexenyl groups used as a main component is a substance of a high molecular weight, it has to be dissolved in an organic solvent. However, the use of an organic solvent creates problems associated not only with the health of the workers but with increased costs since it requires the use of special means for preventing evaporation thereof into the atmosphere.
It has also been proposed to use silicone compositions for forming peelable cured coating films without organic solvents. For example, Japanese Patent Application Laid-Open Number (Kokai) Sho 62-86061 discloses a silicone composition comprising an organopolysiloxane having a low degree of polymerization with hexenyl groups, an organohydrogenpolysiloxane, and a platinum-type catalyst. Japanese Patent Application Laid-Open Number (Kokai) Hei 7-258,606 discloses a silicone composition comprising an organopolysiloxane having a low degree of polymerization with vinyl groups, an organohydrogenpolysiloxcane, and a platinum-type catalyst. Although both these silicone compositions are free of the aforementioned problems associated with the use of organic solvents and decrease in residual adhesiveness of the adhesive substances attached to the peelable cured film at high speeds of peeling from paper (e.g., greater than 50 m/min), the dependency of the resistance to peeling on the speed of peeling becomes low. This may create problems in applications that require certain properties. For example, the aforementioned compositions are not suitable for use in conjunction with peelable paper having different properties on both sides (i.e., for peelable paper having on both sides peelable coating films with difference in peeling resistance).
It is an object of the present invention to provide a silicone composition for forming a release coating with high dependency of the resistance to peeling on peeling speed. Another object is to provide a silicone composition for forming a release coating which is not prone to a decrease in residual adhesiveness of the adhesive substance attached to the cured film.
The present invention relates to a solvent-free silicone composition for forming a cured release coating comprising
(A) 100 parts by weight of a diorganopolysiloxane having in each molecule at least two alkenyl groups and a viscosity of 50 to 5,000 mPaxc2x7s at 25xc2x0 C.;
(B) 3 to 50 parts by weight of an organohydrogenpolysiloxane mixture comprising constituent (b-1) and constituent (b-2) in a weight ratio of 1:0.01 to 1:1, where (b-1) is a diorganopolysiloxane having a viscosity of 1 to 1,000 mPaxc2x7s at 25xc2x0 C. and having both molecular terminals capped with silicon-bonded hydrogen atoms and (b-2) is an organohydrogen polysiloxane having a viscosity of 1 to 1,000 mPaxc2x7s at 25xc2x0 C. and having at least three pendant silicon-bonded hydrogen atoms in each molecular chain; and
(C) a catalytic amount of a platinum-type catalyst.
The present invention relates to a solvent-free silicone composition for forming a cured release coating comprising
(A) 100 parts by weight of a diorganopolysiloxane having in each molecule at least two alkenyl groups and a viscosity of 50 to 5,000 mPaxc2x7s at 25xc2x0 C.;
(B) 3 to 50 parts by weight of an organohydrogenpolysiloxane mixture comprising constituent (b-1) and constituent (b-2) in a weight ratio of 1:0.01 to 1:1, where (b-1) is a diorganopolysiloxane having a viscosity of 1 to 1,000 mPaxc2x7s at 25xc2x0 C. and having both molecular terminals capped with silicon-bonded hydrogen atoms and (b-2) is an organohydrogen polysiloxane having a viscosity of 1 to 1,000 mPaxc2x7s at 25xc2x0 C. and having at least three pendant silicon-bonded hydrogen atoms in each molecular chain; and
(C) a catalytic amount of a platinum-type catalyst.
The diorganopolysiloxane that constitutes component (A) is a main component of the present composition. It is required that this component contain in each molecule at least two silicon-bonded alkenyl groups. The following are examples of the aforementioned alkenyl groups: vinyl, allyl, butenyl, 5-hexenyl, octenyl, and decenyl. Among these, preferred are vinyl and 5-hexenyl. It is recommended that the contents of the aforementioned alkenyl groups provide 0.2 to 10 mole % and preferably 1 to 5 mole % of the total organic groups of component (A). The alkenyl groups can be bonded to molecular terminals, pendant (i.e. bonded to non-terminal silicon atoms), or both. Organic groups, other than alkenyl bonded to silicon atoms are represented by monovalent hydrocarbon groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, or similar alkyl groups; phenyl, tolyl, xylyl, or similar aryl groups; and benzyl, phenethyl, or similar aralkyl groups. Among these most preferred are methyl groups. It is recommended that component (A) have a viscosity of 50 to 5,000 mPaxc2x7s at 25xc2x0 C. and preferably 100 to 2,000 mPaxc2x7s at 25xc2x0 C.
The organohydrogenpolysiloxane mixture comprising component (B) functions as a cross-linking agent. Constituent (b-1) of component (B) is a diorganopolysiloxane having silicon-bonded hydrogen atoms on both its molecular terminals. Organic groups other than hydrogen atoms bonded to silicon can be represented by monovalent hydrocarbons free of aliphatic unsaturated bonds, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, and similar alkyl groups; phenyl, tolyl, xylyl, and similar allyl groups; and benzyl, phenethyl, and similar aralkyl groups. It is recommended that constituent (b-1) have a viscosity of 1 to 1,000 mPaxc2x7s at 25xc2x0 C. and preferably 2 to 500 mPaxc2x7s at 25xc2x0 C.;
Constituent (b-2) of component (B) is an organohydrogenpolysiloxane having at least three pendant (i.e. bonded to non-terninal silicon atoms) silicon-bonded hydrogen atoms in each molecular chain. Silicon-bonded organic groups other than hydrogen atoms can be represented; by monovalent hydrocarbons free of aliphatic unsaturated bonds such as methyl, ethyl, propyl, butyl, pentyl, hexyl, and similar alkyl groups; phenyl, tolyl, xylyl, and similar allyl groups; and benzyl, phenethyl, and similar aralkyl groups. The aforementioned organohydrogen polysiloxane may have a linear, cyclic, branched, or a resin-like molecular structure. It is recommended that constituent (b-2) have a viscosity of 1 to 1,000 mPaxc2x7s at 25xc2x0 C. and preferably 2 to 500 mPaxc2x7s at 25xc2x0 C.
A decrease in the amount of constituent (b-1) in the mixture comprising constituents (b-1) and (b-2) reduces resistance to peeling at high peeling speeds. Therefore it is recommended that the aforementioned constituents (b-1) and (b-2) be used in a weight ratio of 1:0.01 to 1:1 and preferably in a weight ratio of 1:0.01 to 1:0.5, and even more preferably in a ratio of 1:0.05 to 1:0.4.
It is recommended that component (B) be used in an amount of 3 to 50 parts by weight based on 100 parts by weight of component (A). If the content of component (B) is less than 3 parts by weight, the present composition will not cure sufficiently. If, on the other hand, the contents of component (B) exceed 50 parts by weight, this will lead to an increase in resistance to peeling at low peeling speeds and to variations of the peeling resistance with time.
Component (C) is a platinum catalyst which in the present composition is used for acceleration of curing. This component can be represented by metallic platinum such as platinum black, platinum supported on a carrier such as silica or activated carbon, or by platinum-type compounds such as a chloroplatinic acid, an alcoholic solution of chloroplatinic acid, an olefin complex of chloroplatinic acid, a diketone complex of chloroplatinic acid, and a complex of a chloroplatinic acid and 1,3-divinyltetramethyl disiloxane. It is recommended that component (C) be used in a catalytic quantity and preferably in such an amount at which the contents of metallic platinum is within 1 to 1,000 ppm (by weight) based on the, sum of the weights of components (A) and (B).
The present composition is composed of the aforementioned components (A) to (C). However, if necessary to decrease resistance to peeling at low peeling speeds, the composition can be combined with 1 to 20 parts by weight of component (D) which is a dimethylpolysiloxane having a viscosity of 1 to 1,000,000 mPaxc2x7s at 25xc2x0 C. In addition, within the limits not detrimental to the objects of the present invention, the present composition may incorporate various additives conventionally combined with silicone compositions for forming peelable cured coatings such as cure inhibitors, fine powdered silica or similar inorganic: fillers, pigments, heat-resistant additives, powdered organic resins, and dyes. From the point of view of coatability, it is recommended that the present composition have a viscosity of 50 to 5,000 at mPaxc2x7s at 25xc2x0 C. and preferably 50 to 3,000 mPaxc2x7s at 25xc2x0 C.
The present composition can be prepared by uniformly mixing the aforementioned components (A) to (C). If desired, components (A) and (B) can be first premixed and then this mixture can be mixed with component (C).
When applied onto various substrate materials such as plain paper, laminated paper, synthetic resin films, and metal foil with subsequent curing, the present composition forms a coating film peelable from an adhesive substance on the surface of a substrate. Therefore this composition is suitable for use as a release coating on a substrate, in particular as a peelable liner for a paper with different adhesiveness on both sides, e.g., for a double-sided adhesive tape. Since the present composition does not contain an organic solvent, the use of this composition is advantageous from the point of view of environmental safety and improvement in working conditions. Furthermore, by changing the ratio of components (A) and (B), it becomes possible to control resistance to peeling at low peeling speeds, i.e. below 0.3 m/min.